1. Field of the Invention
The present invention generally relates to a color image forming apparatus that forms a color image by superposing a plurality of monochrome images, to a process cartridge, a photosensitive body unit, and a development unit used for the color image forming apparatus, and to a method of adjusting positional deviation of the images. The present invention more particularly relates to a image forming apparatus that adjusts the position of the monochrome images to be superposed, to a process cartridge, a photosensitive unit, and a development unit used therein, and to a method of adjusting positional deviation of the images.
2. Description of the Related Art
Conventionally, color image forming apparatuses form color images by superposing monochrome images of a plurality of colors. Unlike monochrome image forming apparatuses that do not need to superpose images, the color image forming apparatuses, when they fail to precisely adjust the position of monochrome images to be superposed, may have problems such as change in color of line drawings and characters, and mottling. Accordingly, the color image forming apparatuses need to precisely adjust the position of monochrome images to be superposed.
For example, an image forming apparatus that forms color images using a plurality of photosensitive bodies may fail to adjust the position of monochrome images to be superposed in the main scan directions due to various reasons such as change in ambient and inside temperature, and cause positional deviation in the formed color images. Japanese Patent Laid-open Application No. 63-286864 (Patent No. 2642351) discloses an image forming apparatus that can compensates such positional deviation of images.
According to the invention disclosed in the above application, the image forming apparatus is provided with a straight line (reference unit) extending on the transfer belt in the main scan directions and oblique lines-extending oblique to the moving direction of the transfer belt. The reference unit and the oblique lines are detected by sensors. The positional deviation of the oblique lines in the main scan directions is calculated by a CPU based on the reference values stored in a memory and the actual distance between the reference unit and the oblique lines measured by the sensors. At least one of write timing in the main scan directions and write clock is adjusted based on the calculation. Accordingly, the image forming apparatus can compensate for the positional deviation of images due to not only environmental change but also change over time. The image forming apparatus can form high quality color image without color deviation.
Japanese Patent Laid-open Application No. 11-58842 discloses an image forming apparatus that can change the distance between a compensation pattern for measuring color deviation and sensors for detecting the compensation pattern. Accordingly, the image forming apparatus can detect the compensation pattern at high precision.
Such a system forms the compensation pattern for detecting positional deviation of images on the transfer belt, detects the compensation pattern with sensors, and measures the positional deviation of images based on a signal from the sensor. The measured positional deviation is fed back a compensation unit that adjusts the position of the images. In this case, the compensation pattern needs to be high enough in density so that the sensors can detect the compensation pattern.
If the pattern for compensating for the positional deviation of images is sparse, the sensor cannot detect the pattern correctly. Then, the image forming apparatus fails to compensate for the positional deviation of images and consequently forms color images of low quality.
Japanese Patent Laid-open Application No. 7-244412 discloses an image forming apparatus that forms patch images under an image forming condition that is different from an image forming condition under which the images are formed. The image forming apparatus can detect the patch image at high sensitivity. The image forming apparatus disclosed in the above application, however, detects the patch images at high sensitivity in order to improve the quality of images. The image forming apparatus cannot determine whether the patch image is detectible. The image forming apparatus uses a line image, instead of the patch image, as the compensation pattern for measuring color deviation. Accordingly, the image forming apparatus may fail to detect the compensation pattern due to various reasons.
As described above, although the image forming apparatus forms the compensation pattern for compensating for the positional deviation of images, if it fails to detect the compensation pattern, the image forming apparatus cannot compensate for the color deviation, which results in degrading of the image quality. If the image density of the compensation pattern is not high enough for the sensor to detect, the image density needs to be increased.